“When is the right time to burn?” — a question we hear often.

For prescribed fire managers working within the Bay Area’s rich diversity of ecosystems — oak woodlands, redwood and evergreen forests, coastal and inland grasslands — multiple factors, including the unique ecology of vegetation communities, the biology of individual species, and the desired ecological outcomes of the burn help to determine the best “burn windows” for getting good fire on the ground.

Reducing wildfire risk is an added benefit anytime of time of the year.  

FALL 

Oak Woodlands (October – November)  

Oak ecosystems foster tremendous biodiversity and are a hallmark of California landscapes. Oaks are dependent on frequent, low-intensity fire, and in its absence, succumb to encroachment by other tree species, pests wipe out acorn crops, and health declines precipitously to pathogens and stress from lack of available water and light.   

Benefits of this treatment timing include:  

• Reduce the number of acorn pests, making acorns available to germinate into future trees or serve as key resource for wildlife and people  

• Open the understory for new growth  

WINTER 

Redwood & Mixed Evergreen Forests (November – January)  

One important window for burning in redwood and mixed evergreen forests is the winter rainy season during periods with at least 2-3 weeks of drying between rains. The timing allows for moderation of fire intensity without risking damage to the forest canopy. Surface layers are dry enough to burn while the lower layers still hold enough moisture to avoid being consumed.   

Benefits of this treatment timing include:  

• Avoid bird nesting season  

• Reduce heavy fuels to minimize risk of catastrophic wildfire

SPRING

Critical wildfire threat

In the North Bay, mid-January to mid-April is wildflower and songbird nesting season, when we limit the use
of good fire to areas where winter rains have been constant and wildfire threat is so high that reducing fuels prior to wildfire season is worthwhile.

SUMMER 

Invasive Species Reduction: — Medusahead (May – June) and Yellow Starthistle (June – July)

Medusahead is a nonnative winter annual grass with bristly seed heads. Because of its high silica content, livestock avoid grazing medusahead. Thatch accumulation of this invasive weed does not decompose, leading to dense fuel loads. Prescribed burning is 99.6% effective at reducing medusahead by killing next year’s seeds when seedheads are ripe and green.   

Benefits of this treatment timing:  

• Reduce highly flammable invasive species   

• Improve biodiversity in grasslands 

• Improve forage for animals 

Yellow starthistle, a highly invasive and noxious weed, forms dense stands that outcompete native plants. Sharp long spines on flowerheads degrade wildlife habitat and livestock grazing. The plant can cause neurological disorder when eaten, especially in horses. Yellow starthistle must be burned when the plant is bolting.   

Benefits of this treatment timing:  

• Improve wildlife habitat, working lands, and recreational areas  

• Reduce threat to livestock and grazing animals

Coastal Prairies (Aug – Oct)  

Without disturbance, coyote brush and young Douglas fir take over California’s native prairies, the most species-rich grassland in North America, also one of the most threatened. Today, less than one percent of undisturbed coastal prairie remains. Burning during this time allows grass to be dry enough to carry fire and live fuel moisture in coyote brush low enough that it can burn. 

Benefits of this treatment timing:  

• Reduce coyote brush encroachment 

• Rejuvenate native grasses  

The Goldilocks Prescription 

Prescribed fire managers factor in air temperature, wind, humidity, and more before issuing a green light for good fire. Although prescribed fire can be conducted safely and effectively outside of these ranges, common weather and fuels parameters include:

• Air temperature between 60–90 degrees F
• Wind (at eye level) between 3–10 mph
• Relative humidity (RH) between 30–65%
• Fine dead fuel moisture (FDFM) between 6–13%
• Probability of ignition (PIG) — the likelihood of embers igniting in available fuel — between 60–80%